Azarenka had a frustrating first set. At times she looked on song, but on other occasions her decision making and shot selection was off.
Williams was being pushed to the limit to hold her serve, but managed it, then took her break chances on the Azarenka serve.
She led 5-1 at one point, which didn't reflect the closeness of the contest, before converting her third set point to wrap the set 6-3.
But Azarenka had been improving as the match progressed, and shifted the momentum in the second set. Gone were the unforced errors from the first set, and she began to overpower the American at the baseline.
At times she looked like the player that took consecutive Australian Opens in 2012-13, and bested Serena Williams at the peak of her powers.
Williams couldn't get a foothold in the second set; she only won six points on her serve, was broken twice and couldn't convert any of the staggering seven break opportunities.
Azarenka looked the more likely winner as this stage, but Williams, as she has done so often, rediscovered her mojo in the final set.
Azarenka started to struggle on serve, and errors crept in on her forehand wing. Williams grabbed the crucial break in the eighth game, and was good enough to serve out the match, with Azarenka netting a forehand.
As it happened: ASB Classic Day Two